U.S. Pub. No. 2008/0217239, discloses a liquid filter of a composite medium that has a nanoweb adjacent to and optionally bonded to a microporous membrane. The microporous membrane is characterized by an LRV value of 3.7 at a rated particle size, and the nanoweb has a fractional filtration efficiency of greater than 0.95 at the rated particle size of the microporous membrane. According to the disclosure, the nanoweb can be made by electrospinning or electroblowing. According to this disclosure, the composite medium can be used in the form of filter cartridges, in the form of a flat panel or cylindrical unit and can be used in a variety of filtering method applications, such as filtering both gaseous and liquid flows, semiconductor manufacture, and other applications. Examples of polyolefin based microporous films for use as the filtration membrane are described and the specification discloses electroblowing polyamide-6,6 in formic acid to form the nanoweb.
U.S. Pat. No. 7,008,465 discloses a layered filter media that uses a combination of active filtration layers including at least a high efficiency substrate and at least one fine fiber or nanofiber layer to effectively remove dust, dirt and other particulates. Such a substrate type can include HEPA media, fiberglass HEPA, ULPA media, 95% DOP media, melt blown media, electret media, cellulose/meltblown layered media, etc. The nanofiber layer and the high efficiency substrate are selected to obtain a balanced set of properties that permits the user to remove submicron particles efficiently at a relatively low-pressure drop. A high efficiency substrate (either a single layer or a layered substrate structure) has a particulate efficiency exceeding 80% when tested in accordance with ASTM 1215. According to the disclosure the fine fiber of the class of materials can have a diameter of about 0.01 to 5 microns. Such microfibers can have a smooth surface comprising a discrete layer of the additive material or an outer coating of the additive material that is partly solubilized or alloyed in the polymer surface, or both. Materials disclosed for use in the blended polymeric systems are nylon 6; nylon 66; nylon 6-10; nylon (6-66-610) copolymers and other linear generally aliphatic nylon compositions. The fine fibers can be made by electrospinning.
WO 2004/112183, discloses a complex membrane for an electrochemical device such as a lithium secondary battery. The complex membrane includes a micro-porous polyolefin membrane, and a web-phase porous membrane united to at least one side of the micro-porous polyolefin membrane and composed of nanofibers. According to the disclosure, the micro-porous polyolefin membrane is a membrane having at least one layer composed of polyethylene polymer and/or polyethylene polymer, and the micro-porous polyolefin membrane preferably has a thickness of 5 to 50 micron and a porosity of 30 to 80%. Further according to this disclosure the nanofiber preferably has a diameter of 50 to 2,000 nm. The web-phase porous membrane made of nanofibers may be formed on one surface of the micro-porous membranes by directly spinning a polymer solution by means of electrospinning.
Entegris Inc., Japanese Patent Application No. 2008-210063, filed Aug. 18, 2008 discloses and claims a polyamide non-woven fabric manufactured using and electro-spinning method, wherein the fiber diameter is 50 nanometers to 200 nanometers, the 500 mL flow time as defined in the specification is 2-20 seconds, and the 0.144 micron PSL removal rate as defined in the specification is 40-100%. A filter unit having this non-woven fabric is claimed.
JP Publication No. 2007-301436, abstract, discloses an air filter medium that is provided with a sheet-like nanofiber structure layer with which the nanofiber is three-dimensionally entangled, an upstream side porous material layer which integrally overlies the surface of the filtration upstream side of the nanofiber structure layer and a downstream side porous material layer which is integrally laminated on the surface of the filtration downstream side of the nanofiber structure layer. The face which is integrally laminated with the nanofiber structure layer of the upstream side porous material layer and the downstream side porous material layer is flat and smooth with no fluffy projections. The downstream side porous material layer has gas permeability of which the pressure loss is 100 Pa or less at the air flow rate of 1 m/second.
JP Publication No. 2006-326579, abstract, discloses a filter medium that includes a polytetrafluoroethylene (PTFE) porous membrane, an air permeable support material, and a web layer composed of polymer fibers formed by an electrospinning method (charge induction spinning method or electrostatic spinning method). In the filter medium of this invention, an air permeable adhesive layer may be provided adjacently to the web layer. For example, the ranges of the average pore size of the PTFE porous membrane are 0.01 micrometer-5 micrometers. Nylon, polyethylene, and polypropylene electrospun fibers are disclosed.
JP Publication No. 2007-075739, abstract, discloses a filter unit that has a filter medium capturing particles contained in a to-be-filtrated gas and a supporting frame supporting the filter medium. The filter medium has a porous membrane of polytetrafluoroethylene (PTFE), a fibrous filter medium arranged so as to hold the PTFE membrane between the filter medium and a gas permeable supporting material. The fiber constituting the fibrous filter medium has an average fiber diameter of 0.02-15 μm (micron), and the gas permeable supporting material is composed of a fiber of an average fiber diameter of larger than 15 μm. The filter medium is supported with the supporting frame so that the fibrous filter medium lies in the downstream of the flow of the to-be-filtrated gas with respect to the PTFE membrane. According to the disclosure the fibrous filter medium can be electrospun.
WO/2004/069959 discloses filtering of crude resin solution which is a chemically amplified photoresist composition with an acid generator component. According to this disclosure, specific examples of filtration membrane materials include fluororesins such as PTFE (polytetrafluoroethylene); polyolefin resins such as polypropylene and polyethylene; and polyamide resin such as nylon 6 and nylon 66. The specification also discloses passing the crude resist resin solution through a two-stage filter using filtration membranes to effect removal of the polymer and oligomer by-products. In one specific example of the filtering process, the dilute crude resin solution is filtered through a nylon filter as the first filtration step, and the resulting filtrate is then filtered through a polypropylene filter as the second filtration step. A polyethylene filter was also disclosed as being used in this second filtration step.
United States Patent Application No. 20100038307 discloses filtration media including at least one layer of nanofibers having average diameters less than 1000 nanometers with optional porous substrate also referred to as a scrim layer(s). The porous substrates disclosed are spunbonded nonwovens, meltblown nonwovens, needle punched nonwovens, spunlaced nonwovens, wet laid nonwovens, resin-bonded nonwovens, woven fabrics, knit fabrics, apertured films, paper, and combinations thereof. The filtration media are disclosed as having mean flow pore sizes between about 0.5 micron and about 5 micron and are used for filtering particulate matter in liquid. The media are reported to have flow rates of at least 0.055 L/min/cm2 at relatively high levels of solidity and non-diminishing flow rates as differential pressures increase between 2 psi (14 kPa) and 15 psi (100 kPa). This application does not disclose one or more nanofiber layers on a microporous membrane and does not disclose the use of asymmetric microporous membranes.